If one goes down each row nine times (once for each number), notes which columns contain that number and finds no X-Wings, swordfish, etc., is it still possible to find them by checking each column to see what rows contain the number?

If an X-wing occurs in both rows and columns, it is not useful. (For then each candidate occurs only twice in each row and column, and there are no possibilities to eliminate.)

Marty,

To answer your basic question, you must look in both rows and columns. Separately.

The key to these fishes is that they are built on possibilities that occur only twice in a row or column. What I do is to make notations outside the puzzle (next to each row and column) of which possibilities occur only twice.

Then, I look at them to see if they line up to make fishes or coloring chains. (An X-wing is very often also a U- or C-shaped coloring chain connecting four squares.)

I understood Marty to be considering fish of all sizes. No one has suggested that the dual of an X-wing on rows is an X-wing on columns. In general it is not. For example, the dual of an X-wing may be a jellyfish.

I understood Marty to be considering fish of all sizes. No one has suggested that the dual of an X-wing on rows is an X-wing on columns. In general it is not. For example, the dual of an X-wing may be a jellyfish.

I think Keith understands that. All he's trying to suggest is that it's usually easier to spot an X-Wing pattern (4 cells) than it is to find a swordfish (up to 9 cells), or a jellyfish (up to 16 cells). Squirmbags? Those can contain up to 25 cells -- good luck findng one of those, even if you know that it's there.

I've spotted a couple of "jellyfish" in puzzles and felt pretty good about it, too ... until I realized that if I had just rotated my attention by pi/2 I would have found an X-Wing! dcb